1. Field of the Invention
The present invention relates to a detonator for blasting rocks, more particularly to a wireless detonator which utilizes microwaves to cause detonations.
2. Description of the Related Art
Devices which directly activate detonators using received microwaves are well known as conventional radio detonator devices. For example, Examined Japanese Patent Publication No. 61-57558 discloses such a device.
In this device, as shown in FIG. 2, microwave energy received by an antenna 11 is supplied directly to a heating element 13 in a detonator 14 by a transmission circuit 12. Then, the heating element 13 is heated to ignite an igniter, thus triggering the detonator 14.
It is necessary for this device to match the radiation impedance of the antenna 11, the characteristic impedance of the transmission circuit 12, and the impedance of the heating element 13 with each other in FIG. 2. If the radiation impedance of the antenna 11 is not matched with the characteristic impedance of the transmission circuit 12, most of the received microwave energy is reflected at the junction between the antenna 11 and the transmission circuit 12, so that the energy will not be properly carried through. Similarly, if the characteristic impedance of the transmission circuit 12 is not matched with the impedance of the heating element 13, once again, most of the received microwave energy will be reflected at the junction of the transmission circuit 12 and the heating element 13. In both cases, the received microwave energy is not efficiently supplied to the heating element 13. Accordingly, the detonator 14 will not therefore ignite in either case.
A specific description will now be given of the case where a coaxial cable is used as the transmission circuit 12, and a platinum bridge wire serves as the heating element 13 in the device shown in FIG. 2.
The characteristic impedance of a generally used conventional coaxial cable is 50.OMEGA. or 75.OMEGA.. The impedance of a platinum bridge wire is about (0.22+j17) .OMEGA. for microwaves of for example 2.45 GHz. Almost all of the microwave energy is therefore reflected at the junction between the coaxial cable and the platinum bridge wire, so that the energy cannot be efficiently supplied to the platinum bridge wire, causing a misfire of the detonator.
An initiating device disclosed in Japanese Patent Publication No. 63-56480 is shown in FIG. 3. In such a device microwaves received by an antenna 22 are tuned by a tuning circuit 21, which outputs a microwave current. A charging circuit 23 rectifies the microwave current, and charges an igniting capacitor. When the irradiation of the microwaves is completed, a pulse generator 24 generates a trigger pulse. In response to the trigger pulse, an igniter circuit 25 discharges the igniting capacitor of the charging circuit 23 to heat a heating element 26. As a result, the igniter will ignite to trigger a detonator 27.
The impedance matching need not be considered in the above device because the charging circuit 23 rectifies the microwave current. The above-described device however has a complicated structure and requires many circuits.
This initiating device is charged during the irradiation of the microwaves, generates a trigger pulse immediately upon completion of the irradiation, and supplies a current to the detonator 27 to ignite it. The microwaves therefore have to be irradiated for a long time (e.g. 5 to 50 sec). This long irradiation will have an adverse effect on human bodies, animals, and plants, as well as other machinery. To use a detonator of the type described above, some countermeasures should be taken, such as providing workers with protectors or installing protective barriers. Accordingly, the efficiency in blasting work drops.